The present invention relates to a technique for testing or inspecting a property or aspect of a sample such as a wafer. In more detail, the present invention relates to an electron beam apparatus applicable to a defect detection and/or line width measurement of a wafer during a semiconductor manufacturing process and so on, in which electron beams are irradiated to a sample, secondary electrons emitted from the sample and varying according to a property of the sample surface are captured, and image data is created therefrom to evaluate patterns on the sample surface with a high throughput on the basis of the image data. The present invention also relates to an evaluation system and a semiconductor device manufacturing method, both of which utilize the electron beam apparatus. In the present description, the meaning of the term xe2x80x9cevaluationxe2x80x9d of a sample also includes the meaning of xe2x80x9cinspectionxe2x80x9d such as defect detection and line width measurement of a sample.
In semiconductor processes, design rules are now going to enter the era of 100 nm, and the production scheme is shifting from small-kind mass production represented by DRAM to a multi-kind small production such as SOC (silicon on chip). Associated with this shifting, the number of manufacturing steps has been increased, and an improved yield of each process is essential, so that testing for defects caused by the process becomes important.
With the trend of increasingly higher integration of semiconductor devices and finer patterns, a need exists for high resolution, high throughput testing apparatuses. A resolution of 100 nm or less is required for examining defects on a wafer of 100 nm design rule. Also, as manufacturing steps are increased in response to the requirement of higher integration of devices, the amount of testing is increased and thus a higher throughput is required. Further, as devices are formed of an increased number of layers, testing apparatuses are required to have the ability to detect defective contacts (electric defect) of vias which connect lines on layers to each other. While optical defect testing apparatuses are mainly used at present, it is anticipated that electron beam based defect testing apparatuses will substitute for optical defect testing apparatus as a dominant testing apparatus in the future from a viewpoint of the resolution and defective contact testing capabilities. However, the electron beam based defect testing apparatus also has a disadvantage in that it is inferior to the optical one in the throughput. For this reason, a need exists for the development of a high resolution, high throughput electron beam based testing apparatus which is capable of electrically detecting defects.
It is said that the resolution of an optical defect testing apparatus is limited to one half of the wavelength of used light, and the limit is approximately 0.2 xcexcm in an example of practically used optical defect detecting apparatus which uses visible light. On the other hand, in electron beam based systems, scanning electron microscopes (SEM) have been commercially available. The scanning electron microscope has a resolution of 0.1 xcexcm and takes a testing time of eight hours per 20 cm wafer. The electron beam based system also has a significant feature that it is capable of testing electric defects (broken lines, defective conduction of lines, defective conduction of vias, and so on). However, it takes so long testing time that it is expected to develop a defect testing apparatus which can rapidly conduct a test. Further, a testing apparatus is expensive and low in throughput as compared with other process apparatuses, so that it is presently used after critical steps, such as after etching, deposition (including copper coating), CMP (chemical-mechanical polishing) planarization processing, and so on.
A testing apparatus in accordance with an electron beam based scanning (SEM) scheme will be described. An SEM based testing apparatus narrows down an electron beam which is linearly irradiated to a sample for scanning. The diameter of the electron beam corresponds to the resolution. On the other hand, by moving a stage in a direction perpendicular to a direction in which the electron beam is scanned, a region under observation is Two-dimensionally irradiated with the electron beam. In general, the width over which the electron beam is scanned, extends over several hundred xcexcm. Secondary electron beams emitted from the sample by the irradiation of the focussed electron beam (called the xe2x80x9cprimary electron beamxe2x80x9d) are detected by a combination of a scintillator and a photomultiplier (photomultiplier tube) or a semiconductor based detector (using PIN diodes). The coordinates of irradiated positions and the amount of the secondary electron beams (signal strength) are combined to generate an image which is stored in a storage device or output on a CRT (Braun tube). The foregoing is the principle of SEM (scanning electron microscope). From an image generated by this system, defects on a semiconductor (generally, Si) wafer is detected in the middle-of a manufacturing procedure. A detecting speed corresponding to the throughput, is determined by the intensity of a primary electron beam (current value), a size of a pixel, and a response speed of a detector. Currently available maximum values are 0.1 xcexcm for the beam diameter (which may be regarded as the same as the resolution), 100 nA for the current value of the primary electron beam, and 100 MHz for the response speed of the detector, in which case it is said that a testing speed is approximately eight hours per wafer of 20 cm diameter. Therefore, there exists a problem that a testing speed is significantly low in comparison with that in an optical based testing apparatus. For instance, the former testing speed is 1/20 or less of the latter testing speed.
If a beam current is increased in order to achieve a high throughput, a satisfactory SEM image cannot be obtained in the case of a wafer having an insulating membrane on its surface because charging occurs.
As another method for improving an inspection speed, in terms of which an SEM system is poor, there have been proposed SEM systems (multi-beam SEM systems) and apparatuses employing a plurality of electron beams. According to the systems and apparatuses, an inspection speed is improved in proportion to the number of electron beams. However, as a plurality of primary electron beams impinge obliquely on a wafer and a plurality of secondary electron beams are pulled from the wafer obliquely, only secondary electrons released obliquely from the wafer are caught by a detector. Further, a shadow occasionally appears on an image and secondary electrons from a plurality of electron beams are difficult to separate from one another, which disadvantageously results in a mix of the secondary electrons.
Still further, there has been no suggestion or consideration about an interaction between an electron beam apparatus and other sub-systems in an evaluation system employing a multi-beam based electron beam apparatus and thus, at present there aren""t any complete evaluation systems of a high throughput. In the meantime, as a wafer to be inspected becomes greater, sub-systems must be re-designed to accommodate to a greater wafer, a solution for which has not yet been suggested either.
The present invention has been accomplished with a view to obviating the aforementioned problems of prior art and therefore, it is an object of the present invention to provide an evaluation system employing an SEM electron beam apparatus of a multi-beam type and especially an evaluation system capable of improving a throughput of inspection processing.
It is another object of the present invention to provide an SEM electron beam apparatus of a multi-beam type capable of improving not only a throughput of inspection processing but also detection accuracy.
It is still another object of the present invention to provide a method of manufacturing semiconductor devices, according to which a semiconductor wafer can be evaluated by utilizing such an electron beam apparatus or evaluation system as mentioned above irrespective of whether it is in the middle of a fabrication process or upon completion of a fabrication process.
In order to achieve the above objects, the present invention is constituted as follows. That is, a plurality of primary electron beams (multi-beam) are employed to scan a sample in the one-dimensional direction (X direction). The primary electron beams pass through an ExB filter (Wien filter) to impinge perpendicularly upon the surface of the sample, and secondary electrons released from the sample are separated from the primary electron beams by the ExB filter to be pulled obliquely in relation to the axis of the primary electron beams to converge or form an image on a detection system by means of a lens system. Then, a stage is moved in the perpendicular direction (Y direction) with respect to the primary electron beam scanning direction (X direction) to obtain continuous images.
When the primary electron beams pass through the ExB filter, a condition (Wien condition) where the force applied to the electron beams from the electrical field is equal to the force applied from the magnetic field and the directions of the forces are opposite, is set so that the primary electron beams go straight. On the other hand, since the secondary electrons and the primary electron beams advance in the opposite directions, the directions of the forces applied to the secondary electrons from the electrical field and magnetic field are the same and thus, the secondary electrons are deflected from the axial direction of the primary electron beams. As a result, the primary electron beams and secondary electron beams are separated from each other. When electron beams pass through an ExB filter, aberration is larger if the electron beams curve than if the electron beams travel straight. Given that, the optical system of the present invention is designed in such a manner as to cause primary electron beams, which require high accuracy, to go straight and cause secondary electron beams, which do not necessarily require high accuracy, to deflect.
A detection system of the present invention consists of detectors respectively corresponding to primary electron beams, which are arranged such that a secondary electron deriving from its corresponding primary electron beam impinges on the corresponding detector by means of an image-formation system, whereby interaction of signals, that is, cross-talk can be substantially reduced. As a detector, a combination of a scintillator and a photomultiplier, a PIN diode, etc. may be employed. In the electron beam apparatus according to one embodiment of the present invention, sixteen primary electron beams are employed and a beam current of 20 nA having a beam diameter of 0.1 xcexcm is obtained from each of them and therefore, a value of current obtained from the sixteen electron beams in the electron beam apparatus is three times as great as that obtained from the commercially available apparatus at present.
Further, an electron gun for the electron beam apparatus of the present invention uses a thermal cathode as an electron beam source, and LaB6 is employed as an electron emitting material (emitter). Other materials may be used as long as they have a high melting point (low steam pressure at high temperatures) and small work function. In the present invention, two different ways of providing multiple electron beams are employed. One is to pull one electron beam from an emitter (with one protrusion) and pass the electron beam through a thin plate with a plurality of apertures, thereby obtaining a plurality of electron beams. The other is to provide an emitter with a plurality of protrusions and pull a plurality of electron beams directly from the protrusions. The both ways make use of the properties of an electron beam that an electron beam is more easily emitted from the tip of a protrusion. Electron beams from an electron beam source employing other methods, for example, thermal field emission type electron beams may be employed. A thermal electron beam source uses a system for heating an electron emission material to emit electrons, whereas a thermal field emission electron beam source uses a system for applying a high electric field to an electron emission material to emit electrons and further heating an electron beam emission portion to stabilize electron emission.